1. Field of the Invention
The present invention relates to a method and apparatus for changing fuels used in a heating system. More particularly, the present invention is a method and apparatus for changing fuels burned in a heating system without substantially recycling the heating system.
2. Description of the Prior Art
There are many uses for industrial heating systems such as ovens, furnaces and boilers. Many of these heating systems are equipped to burn two fuels, typically natural gas and fuel oil. For example, a school might receive a lower purchase rate for natural gas by agreeing with a natural gas company to switch to oil when the gas company is unable to meet the demands of all its customers. In that case, the school would be notified and would have a certain amount of time to change fuels.
Another example is a boiler used to provide steam for a factory process. When a fuel changeover is required, it may be vital that the fuel changeover is a quick, efficient changeover from one fuel to the other to prevent the loss of steam pressure. The factory demand for steam may continue while the fuel changeover is in progress.
A typical cycle performed by a boiler controller which controls the boiler can be divided into the following major divisions or "states":
Pre-purge, during which the combustion chamber in the boiler is ventilated to remove any fumes from unburned fuel.
Pilot flame establishing period (PFEP) during which a pilot flame is ignited and is monitored to ensure that it is burning.
Main flame establishing period (MFEP) during which the main flame is ignited and is monitored to ensure that it is burning.
Run, during which the boiler runs providing heat as either steam or hot water.
Post-purge, during which the combustion chamber is ventilated for a time interval that allows the main flame to extinguish.
Standby, during which the controller is idle and the burner is off.
In the past, in order to change from one fuel to another in a heating system, the most direct way was by performing a manual operation that included a complete recycling of the burner. A boiler operator would turn off the "burner switch" to cause the burner to shutdown. The shutdown sequence of events typically included the post-purge period after which the burner entered the standby state. While the burner was in the standby state, preparation for burning the alternate fuel would be performed. This involved such tasks as opening some fuel valves and closing others and possibly making some modifications to the burner. Also, since the boiler controller provides safety by testing, among other things, a set of sensors and switch contacts which monitor the fuel system for correct operation, the sensors and contacts for the old fuel would be disabled and those for the new fuel would be enabled.
Following these preparations, the burner switch would then be closed by the boiler operator to restart the boiler via its normal start-up sequence. The start-up sequence typically included a pre-purge period, a PFEP, and a MFEP. Finally, the burner would be running using the new fuel.
This changeover method takes substantial time and manual intervention and wastes heat energy during the post-purge and pre-purge periods. The amount of heat energy lost due to unneeded purging can be a considerable expense over a year's time, and the lost time due to making the adjustments can make the difference between a factory process continuing to run and being temporarily suspended.
Another method for changing fuels which has been used is a custom-made fuel changeover device which is used to automate the fuel changeover process or certain parts of it. However, operation of these devices has not been integrated into the boiler controller. In order to accomplish the changeover, the custom-made fuel changeover devices effectively "trick" the boiler controller into performing certain tasks. Therefore, there is less assurance that the changeover process is being implemented in a correct and safe manner.
For these reasons, there is a need for a changeover process which improves on the manual fuel changeover process by substantially eliminating the need for operator intervention and by minimizing the wasted heat energy and time attributable to the post-purge and pre-purge periods. Also, there is a need for an automatic fuel changeover process which is integrated into the boiler controller's normal operation so potential safety and effectiveness problems which can result by using typical custom-made devices are removed.